Surface Acoustic Wave Scale

ABSTRACT

Apparatus and related methods are provided in a surface acoustic wave (SAW) scale for measuring weight of a load. A processor reads a first frequency of a SAW delay line operating in a first mode. A push oscillator injects a frequency similar to but different than the first frequency in order to cause the SAW delay line to operate in a second mode, and the processor reads a second frequency of the SAW delay line operating in the second mode. A difference between the frequencies is calculated and compared to values in a stored table to determine the first mode at which the SAW delay line was operating. Based on a determination of the first mode and the first frequency, the weight of the load is determined. This determined weight can be used to recalibrate an auxiliary weight sensor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from co-owned Ser. No. 62/878,494,filed on Jul. 25, 2019 and is related to co-owned Ser. No. 15/259,709filed Sep. 8, 2016, now U.S. Pat. No. 10,132,676, Ser. No. 13/742,713,filed Jan. 16, 2013, now U.S. Pat. No. 9,477,638, Ser. No. 09/775,748,filed Feb. 2, 2001, now U.S. Pat. No. 6,448,513, Ser. No. 09/327,707filed Jun. 8, 1999, now U.S. Pat. No. 6,211,473, Ser. No. 08/729,752filed Oct. 7, 1996, now U.S. Pat. No. 5,910,647, and Ser. No. 08/489,365filed Jun. 12, 1995, now U.S. Pat. No. 5,663,531, the completedisclosures of which are herein incorporated by reference in theirentireties.

BACKGROUND 1. Field

This relates to electronic weighing devices, and more particularly to anelectronic weighing device that employs surface acoustic waves tomeasure weight.

2. State of the Art

Precision electronic weighing devices are widely known in the art andthere are many different technologies utilized in these electronicweighing devices. Laboratory scales or “balances” typically have acapacity of about 1,200 grams and a resolution of about 0.1 gram,although scales with the same resolution and a range of 30,000 grams areavailable. The accuracy of these scales is achieved through the use of atechnology known as magnetic force restoration. Generally, magneticforce restoration involves the use of an electromagnet to oppose theweight on the scale platform. The greater the weight on the platform,the greater the electrical current needed to maintain the weight. Whilethese scales are very accurate (up to one part in 120,000), they areexpensive and very sensitive to ambient temperature. In addition, theirrange is relatively limited.

Most all other electronic weighing devices use load cell technology. Inload cell scales, the applied weight bends an elastic member which hasstrain gauges bonded to its surface. The strain gauge is a fine wirewhich undergoes a change in electrical resistance when it is eitherstretched or compressed. A measurement of this change in resistanceyields a measure of the applied weight. Load cell scales are used innon-critical weighing operations and usually have a resolution of aboutone part in 3,000. The maximum resolution available in a load cell scaleis about one part in 10,000 which is insufficient for many criticalweighing operations. However, load cell scales can have a capacity ofseveral thousand pounds.

While there have been many improvements in electronic weighingapparatus, there remains a current need for electronic weighingapparatus which have enhanced accuracy, expanded range, and low cost.

The previously incorporated applications disclose an electronic weighingapparatus having a base which supports a cantilevered elastic memberupon which a load platform is mounted. The free end of the elasticmember is provided with a first piezoelectric transducer and a secondpiezoelectric transducer is supported by the base. Each transducerincludes a substantially rectangular piezoelectric substrate and a pairof electrodes imprinted on the substrate at one end thereof, with onepair of electrodes acting as a transmitter and the other pair ofelectrodes acting as a receiver. The transducers are arranged with theirsubstrates substantially parallel to each other with a small gap betweenthem and with their respective electrodes in relatively oppositepositions. The receiver electrodes of the second transducer are coupledto the input of an amplifier and the output of the amplifier is coupledto the transmitter electrodes of the first transducer. The transducersform a “delay line” and the resulting circuit of the delay line and theamplifier is a positive feedback loop, i.e. a natural oscillator. Moreparticularly, the output of the amplifier causes the first transducer toemit a surface acoustic wave (“SAW”) which propagates along the surfaceof the first transducer substrate away from its electrodes. Thepropagating waves in the first transducer induce an oscillating electricfield in the substrate which in turn induces similar SAW waves on thesurface of the second transducer substrate which propagate in the samedirection along the surface of the second transducer substrate towardthe electrodes of the second transducer. The induced waves in the secondtransducer cause it to produce an alternating voltage which is suppliedby the electrodes of the second transducer to the amplifier input. Thecircuit acts as a natural oscillator, with the output of the amplifierhaving a particular frequency which depends on the physicalcharacteristics of the transducers and their distance from each other,as well as the distance between the respective electrodes of thetransducers.

When a load is applied to the load platform, the free end of thecantilevered elastic member moves and causes the first transducer tomove relative to the second transducer. The movement of the firsttransducer relative to the second transducer causes a change in thefrequency at the output of the amplifier. The movement of the elasticmember is proportional to the weight of the applied load and thefrequency and/or change in frequency at the output of the amplifier canbe calibrated to the displacement of the elastic member. The frequencyresponse of the delay line is represented by a series of lobes. Eachmode of oscillation is defined as a frequency where the sum of thephases in the oscillator is an integer multiple of 2π. Thus, as thefrequency of the oscillator changes, the modes of oscillation movethrough the frequency response curve and are separated from each otherby a phase shift of 2π. The mode at which the oscillator will mostnaturally oscillate is the one having the least loss. The transducersare arranged such that their displacement over the weight range of theweighing apparatus causes the oscillator to oscillate in more than onemode. Therefore, the change in frequency of the oscillator as plottedagainst displacement of the transducers is a periodic function. Thereare several different ways of determining the cycle of the periodicfunction so that the exact displacement of the elastic member may bedetermined.

It is generally known in the art of SAW technology that the frequencyrange in which the losses are the lowest is not necessarily thefrequency range in which the oscillator exhibits the best phaselinearity. From the teachings of the previously incorporatedapplications, those skilled in the art will appreciate that in a SAWdisplacement transducer such as disclosed in the previously incorporatedapplications, better phase linearity provides a more linear relationshipbetween frequency and displacement. In the case of a weighing apparatususing a SAW displacement transducer as described in the previouslyincorporated applications, better phase linearity will result in a morelinear relationship between weight and frequency.

It is known in the art of SAW oscillators that changing the topology ofthe oscillator transmitter and receiver can cause a broader bandwidth ofthe delay line and that a broader bandwidth results in better phaselinearity. It is also known that using a smaller frequency rangeprovides better linearity and that a smaller frequency range can beobtained with a longer delay line. Although these known methods canincrease phase linearity in a SAW oscillator, the frequency range inwhich the best linearity is achieved for a particular oscillator isstill not necessarily the range with the lowest losses.

From the foregoing, those skilled in the art will appreciate that inorder to enhance the accuracy of a SAW displacement transducer such asthat used in a weighing device, it would be desirable to cause the SAWoscillator to oscillate in the range having the best phase linearity.

As disclosed in the previously incorporated applications, weighingaccuracy is affected by temperature. The previously incorporatedapplications disclose a SAW temperature oscillator having a transmitterand receiver on the same substrate. The temperature sensitivity of theload cell disclosed in the previously incorporated applications isapproximately 500 ppm of the weight reading per 1° C. temperaturechange. Accuracy of 100 ppm of the weight reading can be achieved iftemperature is measured to within 0.2° C. which represents a shift ofabout 1 kHz of the SAW temperature sensor. This shift is easy to measurein the short term. The resolution of the SAW temperature sensor is onthe order of 0.001° C. However, the long term stability of the SAWtemperature sensor can drift more than 1 kHz due to many factorsincluding humidity.

In order to overcome some of these issues, co-owned application Ser. No.09/775,748 (U.S. Pat. No. 6,448,513) discloses as one aspect the use ofa “push oscillator” coupled to the delay line for injecting a strong RFsignal at a frequency in the middle of the oscillation mode whichexhibits the best phase linearity. The frequency of the “pushoscillator” is determined experimentally when the scale is calibrated.The RF signal is injected periodically in short bursts. According to asecond aspect of the same patent, the “push oscillator” frequency isgenerated by mixing the temperature oscillator with an adjustable fixedfrequency oscillator. This immunizes the “push oscillator” from theaffects of temperature. According to a third aspect of the same patent,a thermistor is provided for long term temperature stability. The SAWtemperature sensor is periodically calibrated to the thermistor.According to a fourth aspect of the same patent, the SAW oscillators arenot hermetically sealed and the SAW temperature sensor is used tocorrect the displacement sensor for changes in environmental conditionssuch as humidity.

Even with these improvements, SAW scales still do not meet certaincriteria that are desirable for high accuracy scales. For example, whilethe zero stability of such SAW scales is in the desirable range of1:50,000 to 1:100,000 (for a temperature range of 10° C.-40° C.), thestability of the span parameter (the weight reading after having zeroedthe scale) is typically as low as around 1:10,000. It is desirable thatthe span parameter be in the same range (i.e., 1:50,000 to 1:100,000) asthe zero stability.

The main cause of this problem is the fact that the process ofdetermining the load for the scale consists of measuring the frequencyof the SAW transducer under two conditions—first without load (the zerovalue) and the other under load from the platform (the weight value). Aquality of the SAW scale is that zero stability and the span parameterstability for these two frequencies depends on their values within thepass band of the transducer. The zero stability for every point insidethe pass band is very similar, but does have slight variations. As anexample, without any load on the platform, the frequency of the delayline oscillator could be 92.9 MHz. Under load it could be 93.1 MHz. Inthis example the span parameter for a single mode is 200,000 Hz (0.2Mhz). If the scale utilizes multiple modes, the span parameter iseffectively 200,000 Hz times the number of modes of the scale. For fivemodes, the span parameter of the scale is effectively 1.0 MHz.

The span parameter is also dependent upon temperature. For example, forthe exemplary spam parameter described above, the frequency of the delayline oscillator without load for two different temperatures can change(i.e., drift) by 1000 Hz, and the frequency of the delay line oscillatorunder load for the same two temperatures can change (i.e., drift) by1050 Hz. This is a difference of 50 Hz and is referred to as absolutespan drift. In this example, the relative span drift (absolute spandrift/span parameter) is 50 Hz/200,000 Hz (1:4000) (for a single mode),which is considered to be a poor result for a high accuracy scale. Ifthe scale utilizes five modes, the absolute span drift (50 Hz) will bethe same, but the full range will be five times larger and as a resultthe relative span drift will drop to 1:20,000, which is still higherthan desired. In addition, this error will appear as a discontinuity andas a linearity distortion at the points of the border between modes.

In addition, given the wide range of temperatures under which industrialscales operate, −20° C. to +60° C., there is the potential formeasurement error due to the mismatching of coefficients of thermalexpansion (CTE) between the SAW transducer substrate and the material ofthe load cell. The transducer substrate is bonded to the load cell usinga holder which is made from the same material as the remainder of theload cell; typically, a suitable alloy of aluminum. The transducersubstrate and the holder material have significantly different CTEswhich will subject the bonding line of the materials to a thermalstress. If the temperature changes significantly, the thermal stressbetween the materials, including the bonding line, causes some change onthe zero reading of the scale which is determined by the exact positionof the transducer substrate. Because the bonding material has some levelof hysteresis and non-repeatability under stress, the shift of the zeroreading can be very unpredictable. Various methods are known for bondingmaterials with mismatched CTEs, including high temperature or pressurebonding, including brazing or diffusion, or machining operations,including drilling holes or riveting. However, the transducer substratematerial is not suitable for these kinds of operations because offragility and high temperature concerns.

Co-owned U.S. Pat. No. 9,477,638 resolved various of these issues. Byway of example, in embodiments, the SAW transducer was fabricated on athin lithium niobate piezosubstrate and attached by glue or a bondingagent directly to a metal holder of selected thickness or via anintermediate bonding plate having a coefficient of thermal expansion(CTE) that closely matched that of the piezosubstrate so that thepiezosubstrate and holder can bend together without overstressing thebonding layer. Also, by way of example, an auxiliary sensor was providedto ascertain the operating mode of the SAW scale is adjusted(calibrated) by comparing the reading of the SAW sensor and the readingof the auxiliary sensor. Further, by way of example, the SAW scale wascapable of automatically recalibrating to account for changingenvironmental factors. In this aspect, during a period of time whenthere was no change to weight applied to the scale, readings of SAWtransducers which relate to weight indications and environmental factorindications were taken for each one of two adjacent operating modes ofthe scale, and two calibrated weight calculations were made utilizingthose readings. The difference in calibrated weight calculations wasthen related to a variable utilized to transform the readings intoweights, which was updated, thereby recalibrating the scale.Recalibration in this manner significantly reduced span drift andenhanced linearity.

Despite all of the advances, there still remain situations where theauxiliary sensor drifts sufficiently to cause ambiguity as to the modein which the SAW sensor is functioning. For example, when the scale isnot used for a very long period of time and is then turned on, theauxiliary sensor may not be helpful in identifying what mode the SAWsensor is functioning. Or, if the scale is moved and is jostled, theauxiliary sensor may not provide necessary information to identify theSAW sensor mode.

SUMMARY

According to one aspect, a method and apparatus are provided forrecalibrating the auxiliary sensor of a SAW scale by storing thefrequency differences between adjacent resonance modes of the SAW scaleupon initial (factory) calibration, and by determining the operatingoscillation frequency of the SAW delay line when the scale is poweredup. From the determination of the operation oscillation frequency uponpower up, the mode in which the SAW delay line is working may bedetermined, and the exact weight on the platform may be determined fromthe SAW sensor. The weight value may then be used to recalibrate thesecondary sensor and eliminate drift that may have occurred since themost recent previous calibration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side elevation view of an exemplary embodiment.

FIG. 1a is an enlarged schematic plan view of a first transducer.

FIG. 1b is an enlarged schematic plan view of a second transducer.

FIG. 2 is an enlarged schematic side elevation view of a transducerhaving anti-reflection structure.

FIG. 3 is an enlarged schematic side elevation view of a pair oftransducers according to one embodiment.

FIGS. 4 and 5 are graphs of a portion of a frequency response curve foran exemplary delay line according to the invention showing modes ofoscillation and phase shifting.

FIG. 6 is a simplified schematic diagram of circuits used in a weighingdevice.

FIG. 7 is a graph plotting load against a frequency function and showing“modes” of operation for an exemplary embodiment.

FIG. 8 is a graph showing exemplary changes in the load-frequencyfunction plot due to environmental affects.

FIG. 9 is a flow chart implemented by the microprocessor of FIG. 6 forautomatic recalibration.

FIG. 10 is an enlarged schematic side elevation view of a pair oftransducers according to another embodiment.

FIG. 11 is a schematic side elevation view of a piezosubstrate on aholder therefor.

FIG. 12 is a front perspective view of an assembly of the piezosubstrateof the transducer to the holder of FIG. 11 in a manner which reducesthermal stress on the assembly.

FIG. 13 is a longitudinal section view across line 13-13 in FIG. 12.

FIG. 14 is a front perspective view of the holder shown in FIG. 12.

FIG. 15 is a longitudinal section view of another embodiment of anassembly of the piezosubstrate to the holder in a manner which reducesthermal stress on the assembly.

FIG. 16 is a longitudinal section view of yet another embodiment of anassembly of the piezosubstrate to the holder in a manner which reducesthermal stress on the assembly.

FIG. 17 is a flow-chart of a method of recalibrating an auxiliary orsecondary sensor.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring now to FIGS. 1, 1 a, and 1 b, an electronic weighing apparatus10 includes a base 12 which supports a cantilevered elastic member 14having a cut-out 15, and upon which a load platform 16 is mounted. Thecut-out 15 is provided with two opposed posts 17, 19 upon which arerespectively mounted a first piezoelectric transducer 20 and a secondpiezoelectric transducer 22. As is standard in the art, post 17 isrigidly coupled to base 12, and post 19 is coupled to the elastic member14. The posts 17, 19 serve to locate the transducers 20, 22 at thecenter of the elastic member 14 and to mechanically couple thetransducers to opposite ends of the elastic member 14. According to oneaspect, an auxiliary displacement sensor 18 is also provided. In oneembodiment auxiliary displacement sensor 18 is a capacitive sensor withone plate 18 a coupled to post 17 and the elastic member 14 serving asthe other plate. If desired, a second plate could be attached to theelastic member. Alternatively, plates may be located such that one plateis stationary and the other can move as a function of the weight onplatform 16. Other embodiments of an auxiliary displacement sensorinclude one or more strain gauges coupled to the elastic member 14, orone or more inductive members. As will be discussed in detailhereinafter, according to one aspect, the auxiliary displacement sensoris calibrated initially by the manufacturer, and the weighing apparatusis provided with a mechanism to automatically recalibrate the auxiliarydisplacement sensor over the life of the apparatus 10.

The first transducer 20 includes a substantially rectangularpiezoelectric substrate 20 a and a pair of electrodes 20 b imprinted onthe substrate at the upper end thereof. The second transducer 22includes a substantially rectangular piezoelectric substrate 22 a and apair of electrodes 22 b imprinted on the substrate at the lower endthereof. In one embodiment the substrates are made of lithium niobate.The transducers are arranged with their substrates substantiallyparallel to each other with a small gap “g” between them. The electrodes22 b of the second transducer 22 are coupled to the input of anamplifier (not shown) powered by a power source (not shown) and theoutput of the amplifier is coupled to the electrodes 20 b of the firsttransducer 20. The circuit arrangement is the same as shown in thepreviously incorporated application Ser. No. 08/489,365, previouslyincorporated herein by reference. The resulting circuit is a positivefeedback loop natural oscillator, a “delay line”. The output of theamplifier generates an alternating voltage in the electrodes 20 b of thefirst transducer 20 which generates a surface acoustic wave (“SAW”) 26which propagates along the surface of the first transducer substrate 20a away from its electrodes 20 b. Since the substrate 20 a of the firsttransducer 20 is relatively close to the substrate 22 a of the secondtransducer 22, an oscillating electric field which is induced as aresult of the SAW waves 26 in the piezoelectric substrate 20 a is ableto in turn induce similar SAW waves 28 on the surface of the secondtransducer substrate 22 a which propagate in the same direction alongthe surface of the second transducer substrate toward the electrodes 22b of the second transducer 22. The induced waves 28 in the secondtransducer 22 cause the electrode 22 b of the second transducer 22 toproduce an alternating voltage which is provided to the input of theamplifier. As long as the gain of the amplifier 24 is larger than theloss of the system, the circuit acts as a natural oscillator with theoutput of the amplifier having a particular frequency which depends onthe physical characteristics of the transducers and their distance fromeach other, as well as the distance between the respective electrodes ofthe transducers. In particular, the frequency of the oscillator isdirectly related to the time it takes for the SAW 26 to propagate fromthe electrodes 20 b to the electrodes 22 b.

According to certain embodiments described in more detail below, the SAW26 has a wavelength of approximately 100-200 microns at 20-50 MHz. Inorder to limit loss in the system, the gap “g” between the substrates ofthe first and second transducers is kept small. In one embodimentdescribed below, the gap is 10-20 microns. With such a gap, anoscillating system can typically be generated if the amplifier 24 has again of at least approximately 25 dB. It will be appreciated that when aload (not shown) is applied to the load platform 16, the free end of thecantilevered elastic member 14 moves down and causes the secondtransducer 22 to move relative to the first transducer 20. Inparticular, it causes the electrodes 22 b of the second transducer 22 tomove away from the electrodes 20 b of the first transducer 20. Thisresults in a lengthening of the “delay line”. The lengthening of thedelay line causes a decrease in the frequency at the output of theamplifier. The displacement of the elastic member is proportional to theweight of the applied load and the frequency or decrease in frequency atthe output of the amplifier can be calibrated to the distance moved bythe elastic member.

It will be appreciated that locating the transducers at the center ofthe elastic member compensates for any torque on the member which wouldexhibit itself at the free end of the member. This results in animproved accuracy as compared to the weighing instrument disclosed inU.S. Pat. No. 5,663,531. Depending on the application (e.g. maximum loadto be weighed), the elastic member is made of aluminum or steel. In oneembodiment, the elastic member exhibits a maximum displacement of 0.1 to0.2 mm at maximum load.

Reflected waves may occur on both piezosubstrates. Reflected wavesinterfere with the received signal. The interference causes an increasein non-linearity. FIGS. 2 and 3 show an embodiment of anti-reflectionstructures.

Turning now to FIGS. 2 and 3, one embodiment of transducers 20, 22(numbered 120, 122) is shown. FIG. 2 illustrates the features oftransducer 120 which is substantially identical to transducer 122. FIG.3 illustrates the transducers mounted on holders 117, 119 to the posts17, 19 of the elastic member 14 of FIG. 1. As shown in FIG. 2, thetransducer 120 includes a lithium niobate substrate 120 a withelectrodes 120 b attached thereto by glue 123. The ends 120 c, 120 d ofthe substrate are tapered and polyurethane dampers 121 a, 121 b areplaced at the ends to minimize reflection of the SAW waves.

According to one aspect, the lithium niobate substrate 120 a and/or theholder 117 is/are adapted to reduce the stress on the glue when theambient temperature changes significantly, thereby significantlyreducing or eliminating random hysteresis effects and resulting zeroshifts. In one embodiment the lithium niobate substrate 120 a is adaptedby providing a substrate of thickness between 0.25 mm and 0.1 mm. Thismay be done by grinding down or otherwise reducing the thickness of athicker (e.g., 0.5 mm) lithium niobate piezosubstrate. The holder 117 isselected to be at least ten times the thickness of the piezosubstrate.In another embodiment, the holder selected to be between 1 and 3.5 timesthe thickness of the piezosubstrate such that the piezosubstrate andholder bend together like a bimetallic strip when the ambienttemperature changes significantly. Thus, by way of example only, if thepiezosubstrate is 0.5 mm thick, the holder is selected to be betweenapproximately 0.5 mm and 1.75 mm thick, and if the piezosubstrate is 0.2mm thick, the holder is selected to be between 0.2 mm and 0.7 mm thick.

Turning now to FIGS. 10 and 11, according to another embodiment of thetransducer, the lithium niobate substrate is adapted to have reducedstress on the glue bond when the ambient temperature changessignificantly, and consequently significantly reduced or eliminatedrandom hysteresis effects and resulting zero shifts. A holder 317 formounting the piezosubstrate 320 to arm 17 is provided. The holder 317includes a support 330 and a substrate mounting face 338. The support330 includes two holes 332, 330, and extends into a slot 17 a in the arm17 at which it is connected with mounting screws 336 to the arm 17. Thesubstrate mounting face 338 is oriented substantially orthogonal to andvertically offset relative to the end of the support 330. In oneembodiment, the holder 317 is machined from a unitary piece of material,e.g., metal, and more particularly, for example, the same aluminum alloy2024 of which the remainder of the load cell is constructed. Aluminumalloy 2024 has a coefficient of thermal expansion (CTE) of approximately25 ppm/° C., whereas the CTE of 128° YX lithium niobate substrate (inthe direction of SAW propagation) is approximately 15 ppm/° C.Ordinarily, when materials of such different CTEs are bonded to eachother and they undergo temperature changes, they are subject to thermalstress. If the temperature changes significantly, the thermal stresscould cause some change on the zero reading of the scale. The amount ofchange is determined by the stress displacement of the substrate whichis affected by the internal stress applied to the substrate by thebonding material between the substrate and the holder. Because thebonding material has some level of hysteresis and non-repeatability, theshift of the zero reading can be unpredictable. In order tosignificantly reduce or eliminate the potentially significant thermalstress that can result along a bonding interface, the substrate can becoupled to the holder as follows.

In the embodiment of FIGS. 12 through 14, the holder 317 is machinedwith a cantilevered beam 340 having a free end 342 and a rotation point344 at which the beam is connected to the mounting face 338 of theholder and about which the beam is permitted relatively free rotation.The mounting face 338 has a first recess 346 for receiving dampingadhesive 348 between and in contact with the piezosubstrate 320 and theholder, such as RTV silicone adhesive, to suppress parasitic bulk waves.In one embodiment the adhesive is a relatively soft material so that itwill not introduce thermal stress. Suppression of bulk waves preventsdistortion of SAW line linearity and therefore maintains linearity ofthe scale. Linearly spaced from the first recess 346 is a smaller secondrecess 350 acting a reservoir for bonding agent overflow, as explainedfurther below. If desired or necessary, additional front face recessescan be formed for receiving damping agent or bonding agent overflow.

The substrate 320, provided with electrodes 320 b, is bonded with abonding agent to the face 338 at only two points: the first point 352 ator adjacent at the free end of the cantilevered beam and the secondpoint 354 just to the far side of the second cavity 350. Because thepoints of bonding are small, the bonding will not introduce thermalstress. Any overflow of bonding agent at one end will flow to the sidesof the beam 340, whereas any additional agent at the second end of thesubstrate will enter the second cavity 350 to ensure that the substrate320 seats close to and with planarity to the holder face 338. Then, whenthe piezosubstrate 320 stretches or shrinks due to temperature changes,the beam 340 will bend about the rotation point with the piezosubstrate.Because the expansion or contraction of the piezosubstrate is in a rangeof at most several microns, and the length of the beam 340 is severalthousand microns, it can be assumed that the free end 342 of the beam ismoving in the direction of SAW propagation.

Temperature changes that cause the piezosubstrate 320 to expand orshrink a different amount than the face 338 of the holder will not causesignificant stress to be applied to the bonding points 352, 354 becausethe free end 342 of the beam allows thermal expansion of thepiezosubstrate without resistance. This can be confirmed, as follows.

First, for comparison purposes, assume a piezosubstrate which is 10 mmin length, 2 mm wide, and 0.5 mm thick bonded to an aluminum substrateholder with a thin layer of bonding agent between the two. Also, assumea holder substrate of 3.3 mm. Because the substrate of the holder issignificantly thicker than the piezosubstrate, a change in temperatureof approximately 10° C. can cause the piezosubstrate to be stretched byapproximately 1 micron. The force applied to the piezosubstrate throughthe bonding adhesive is calculated as F=SeE, where S is cross-sectionalarea of the piezosubstrate, which is 1 mm², e is strain, which is0.0001, and E is Young's modulus of the piezosubstrate, which is 21000kg/mm². This corresponds to approximately 2000 grams force.

Now consider the force under the modified assembly. The beam 340 is 1 mmthick in the plane of bending at the rotation point 344, 3.3 mm wide(the thickness of the face 338) and 7.6 mm in length (between therotation point 344 and the free end 342). When the temperature changesby 10° C., the free end 342 of the beam which is bonded to thepiezosubstrate 320 will yield by the same 1 micron and the forcerequired for this amount of bending can be estimated as F=3dEI/L³, whered is the displacement of the free end, which is 1 micron, E is theYoung's modulus of the aluminum alloy, which is 7000 kg/mm², I is amoment of inertia of the beam, which is 0.275 mm⁴, and L is a length ofthe beam, which is 7.6 mm. This corresponds to approximately 14 grams offorce, less than 1 percent of the force to which the piezosubstrate issubject when adhesive bonded along its entire surface to the holder.Thus, almost all of the stress applied to the bonding points 352, 354 iseliminated with the described assembly. Moreover, the beam 340 is notsubject to hysteresis or non-repeatability, because it is manufacturedas part of the holder 317 with no special bonding to the holder.

Turning now to FIG. 15, another embodiment of an assembly avoids anybonding agent directly between the piezosubstrate and the plate holder,which have very different CTEs from each other. It is known that the CTEfor 128° YX lithium niobate is very different for its different axes.The length of the piezosubstrate, extending along the axis of SAWpropagation, is much bigger than the width. Therefore, it is moreimportant to achieve the best match of the respective CTEs of thepiezosubstrate and the holder in the direction of SAW propagation,rather than in other directions. The CTE in this direction is CTE of 15ppm/° C. In accord with this embodiment, an intermediate plate 460having a CTE approximating that of the piezosubstrate 420 in thedirection of SAW propagation is bonded to the face 438 of the holder417, and the piezosubstrate 420 is bonded to the intermediate plate 460using a suitable bonding material 466, e.g., epoxy adhesive. Oneexemplar material for the intermediate plate 460 is stainless steelA316, which has a CTE of 16 ppm/° C. Stainless steel A316 also can bemachined and endure high temperature and pressure. In one embodiment theintermediate plate 460 is relatively thick, e.g., 0.06 inch, to maintaina high degree of flatness during machining operations. Importantly, theintermediate plate 460 is bonded to the face 438 of the aluminum alloyholder 417 without an adhesive bonding agent. Rather, to “bond” theintermediate plate 460 and the face 438 of the metal holder 417, twopins 462, 464 are inserted through the plate and holder to stablyconnect them together. The pins 462, 464 are offset to be located to oneside of the adhesive bonding line 466 between the piezosubstrate 420 andthe intermediate plate 460, and in one embodiment are spaced relativelyclose together. In this manner, as the piezosubstrate 420 stretches orshrinks due to temperature changes, the intermediate plate 460 willsimilarly stretch or shrink without resulting in overstressing theadhesive bond 466 between the piezosubstrate 420 and the intermediateplate 460. Further, all thermal stress is limited to the small space 468between the pins as the SAW transducer is subject to thermal changes.

Using this assembly, a scale was tested for ranges of temperatures +6°C. to 20° C. and then to 50° C. and then back to between +6° C. to 20°C. several times. The thermal cycling showed very low hysteresis andnon-repeatability regarding zero shift. The scale meets NTEPrequirements for 1:15000.

It is noted that an intermediate plate 460 of large thickness such as inthe assembly of FIG. 15 can be subject to relatively high levels ofthermal stress at the pins 462, 464. Thus, referring to FIG. 16, anotherassembly embodiment is provided in which such thermal stress isessentially eliminated even under harsh cycling conditions. The SAWtransducer is fabricated on a lithium niobate piezosubstrate 520, thepiezosubstrate 520 is bonded with a bonding agent 566 to an intermediateplate 560 having a CTE that closely matches the piezosubstrate (in thedirection of SAW propagation), and the face 538 of the metal holder 517is machined with a cantilevered beam 540 that is permitted relative freerotation around a rotation point at which the beam is connected to themain body of the holder. The intermediate plate 560 is bonded to theface 538 of the metal holder with two pins 562, 564, one extending intothe cantilevered beam 540 on one side of the piezosubstrate 520 and oneextending in the face 538 on the other size of the piezosubstrate. Asdiscussed in reference to the embodiment shown in FIGS. 12 through 14,it is appreciated that by coupling the intermediate plate 560 at one endto the cantilevered beam and at its other end of the face 538, withoutbonding agent extending along the interference therebeneath, thermalstress is effectively eliminated from the assembly.

As mentioned above and in the previously incorporated application, thedelay line may oscillate in more than one mode and within each mode, thegain will vary as the frequency changes. Referring now to FIGS. 4 and 5,in the idle state, with no weight applied to the scale, the delay linewill oscillate at a frequency “f” which is shown in FIG. 4 as the pointhaving the most gain (least loss). The optimal gain area of the graph ofFIG. 4 is shown in the shaded area surrounding f and represents a rangeof ±100 Khz, for example. This area is considered optimal because it isthe area of least loss. However, it is not necessarily the area of bestphase linearity. After experimenting, it may be discovered thatoscillation in a different mode, e.g. the shaded area of FIG. 5, willproduce better phase linearity. According to one aspect, the oscillatoris forced to oscillate in the mode of best phase linearity by injectinga strong RF signal having a frequency at the midpoint of the desiredmode of oscillation. The RF signal is injected by a “push oscillator”coupled to the SAW wave receiver as described in more detail below withreference to FIG. 6. According to one embodiment, the RF signal has astrength of approximately 100 my as compared to the SAW oscillator'sstrength of approximately 10 my. The RF signal may be injected for ashort time (as short as 0.01 seconds) before each weight measurement.

As mentioned above, and described in detail in the previouslyincorporated applications, the effects of temperature can be furthercorrected by providing a separate SAW temperature sensor on the samesubstrate as one of the displacement transducers. According to oneembodiment, the SAW displacement oscillator operates at 55 MHz and theSAW temperature oscillator operates at 57 MHz. According to anotheraspect described in more detail below with reference to FIG. 6, thetemperature oscillator is used in conjunction with an adjustable 2 MHzoscillator and a mixer to produce the “push oscillator” frequency andautomatically adjust the “push oscillator” frequency for temperaturechanges.

As seen in FIG. 6, an exemplary circuit 200 includes the displacementSAW transducer formed by the transmitter 122 b on the substrate 122 andthe receiver 120 b on the substrate 120 coupled to each other by theamplifier 202. In addition, the circuit includes a temperature SAWtransducer formed by the transmitter 124 and receiver 126 on thesubstrate 122 coupled to each other by the amplifier 204. The output ofamplifier 202 is a frequency Fw which varies according to displacementof the substrates relative to each other, which is an indication ofweight when the transducers are arranged as shown in FIG. 1. Accordingto one embodiment, the frequency Fw is nominally 54 MHz. Fw will alsovary according to temperature. The output of amplifier 204 is afrequency Ft which varies only according to temperature and humidity andwhich is nominally 57 MHz. The frequencies Fw and Ft are mixed(subtracted) at the mixer 206 to produce a nominal frequency of 3 MHzwhich varies according to weight and which is temperature compensated.The output frequency of the mixer 206 is input to a microprocessor 208which calculates weight as described in the previously incorporatedapplications and displays the weight on display 210. According to thisembodiment, the output Ft of amplifier 204 is also mixed via mixer 212with a 54 MHz signal from oscillator 214 to produce a signal which isnominally 3 MHz and which varies only with temperature and humidity. Thesignal Fw-Ft provides a temperature adjusted weight signal whichaccounts for the affects of temperature on the SAW oscillators. It doesnot compensate for temperature effects on the Youngs modulus of theelastic member (14 in FIG. 1). The signal output from mixer 212 is apure temperature indicator and is used to adjust the weight calculationfor the effects of temperature on the Youngs modulus of the elasticmember.

According to one aspect, a “push oscillator” is formed from anadjustable oscillator 216, a mixer 218, and a modulator 220. Theoscillator 216 has a nominal frequency of 2 MHz which is mixed via themixer 218 with the output of amplifier 204 to produce an outputfrequency Fi which is (Ft—approx. 2 MHz). This frequency Fi is used toindex the modulator 220 which produces the “push oscillator” output tothe input of amplifier 202. As shown in FIG. 6, the modulator 220 andthe oscillator 216 are both coupled to the microprocessor 208. Themicroprocessor 208 is programmed to periodically activate the modulator220 to inject the push frequency as described above. In addition, themicroprocessor advantageously is utilized to adjust the oscillator 216to determine the frequency of the “push oscillator”. The oscillator 216may be initially adjusted via a simple variable resistor or variablecapacitor. However, it is further adjusted by the microprocessor duringoperation of the scale. One of these advantages is that themicroprocessor can adjust the oscillator 216 to produce the phaseshifting described in the previously incorporated applications. Inaddition, it can be used to produce much larger frequency shifts thanwere possible in the previously incorporated applications. This resultsin more accurate determinations of which weight range the scale is in.As described in the previously incorporated applications, the oscillatoroperated as a periodic function where the same frequencies were repeatedover different weight ranges. A phase shift of ±π was used to determinewhich weight range the scale was operating in. As the weight increased,the same phase shift produced a larger frequency shift (because of theincreased length of the delay line) and the frequency shift could beused to determine the weight range. However, under some circumstances,the phase shift resulted in a frequency shift which was too small toaccurately determine. In one embodiment the push oscillator can be usedto produce ±4π phase shifts.

As mentioned above, in one embodiment the oscillator 216 is initiallyadjusted with a variable resistor or variable capacitor to ensureoscillation on the mode of best phase linearity. Initial calibration isperformed as follows: Known weights are placed on the scale and thefrequency of the oscillator output is determined for different weightsand the modes of oscillation are noted. The push oscillator is tuned tooperate in one mode and experiments are conducted to measure linearity.The experiments are repeated for each mode. The push oscillator is thentuned to push to the mode of best linearity.

Also, as mentioned above, the auxiliary displacement sensor 18 isinitially calibrated by placing known weights on the scale and providing(capacitive) readings. These readings are correlated by themicroprocessor 208 to the readings of SAW delay line so that the mode inwhich the scale is operating can be determined. In other words, and aswill be discussed in more detailed below, different loads on the pan orplatform 16 can produce the same frequency response in the SAW delayline such that a weight determination cannot be made unless the mode isknown. Because the auxiliary displacement sensor 18 has a one-to-onecorrespondence between output readings and weight (i.e., it does nothave multiple modes), it provides information to the microprocessor fromwhich a determination is made as to what mode the system is in. However,it will be recognized by those skilled in the art that the auxiliarysensor will typically be much less accurate and stable than the overallSAW scale. However, for a SAW scale with, e.g., 6 modes and 0.2 mmdisplacement, the stability of, e.g., a capacitive sensor with 10%change of its capacitance under 100% load should be 0.5% for thetemperature range −20° C. to 50° C.

In order to achieve this level of stability, according to one aspect,from time to time (e.g., regularly, such as every day or month, butpossibly at random times), the reading of the SAW sensor and the readingof the auxiliary sensor are compared, and the reading of the auxiliarysensor is adjusted (recalibrated) to match the reading of the SAW scale.The reading of the SAW sensor and auxiliary sensor may be done whetheror not there is weight on the scale. This comparison and adjustmenttechnique is effective because the primary sensor of a SAW scale, theSAW delay line oscillator, has very good stability (relative drift onthe order of 10 parts per million of its central frequency in one year),whereas the reading of the auxiliary sensor is not as stable, but driftsrelatively slowly. This means that after proper calibration of the SAWscale auxiliary sensor, it will always show the correct mode number inspite of its inherent instability. This will maintain the high overallaccuracy of the scale.

Turning now to FIGS. 7-9, a mechanism for automatic recalibration of thescale is provided that reduces span drift. In this aspect, during aperiod of time when there is no change to a weight applied to the scale(i.e., the weight is static), (e.g., when there is no weight beingapplied to the scale, or when the weight being applied is steady),readings of SAW transducers that relate to weight indications andenvironmental factor indications are taken for each one of two adjacentoperating modes of the scale, and two calibrated weight calculations aremade utilizing those readings. The difference in calibrated weightcalculations is then related to a variable utilized to transform thereadings into weights, which is updated, thereby recalibrating thescale. Recalibration in this manner significantly reduces span drift andenhances linearity.

More particularly, a load-frequency function graph seen in FIG. 7, wherethe x-axis indicates weight (P), and the y-axis is a frequency functionX(f)=Ft/Fw, with Ft being the SAW reference sensor frequency indicationoutput by amplifier 204 of the delay line oscillation loop 124, 126,204, and Fw being the SAW weight frequency indication output byamplifier 202 of the delay line oscillation loop 120 b, 122 b, 202. Aswill be appreciated by those skilled in the art, FIG. 7 presents asaw-tooth function, where the same frequency function X(f) can representmultiple weights P. This multiple to one mapping is indicative ofmultiple “modes”. In FIG. 7, three modes (−1, 0 and 1) are shown,although the SAW scale can include four, five, six or more modes.

It can be shown that the relationship between the frequency functionX(f) and the weight P may be expressed according to the equation:

P=[W*(X−X0)*{(N0+S)/N0}*AW*(t−t0)]+[Dp*S*AP*(t−t0)]  (1)

where W is the inverse of the slope of the zero mode;

-   -   X is the value of X(f) at weight P=P;    -   X0 is the value of X(f) at weight P=P0 (at initial calibration,        typically when there is no weight on the scale);    -   S is the number of the mode (e.g., S= . . . , −2, −1, 0, 1, 2, .        . . );    -   N0 is the number of wavelengths between the transducers of the        SAW delay line at P=0;    -   Dp is a beat period for the scale (i.e., the distance along the        x-axis between the saw teeth, which equates to the amount of        weight required to cause the scale to change modes) and which is        constant for the scale;    -   AW is the temperature coefficient of the inverse slope, which is        determined during an initial calibration process by changing the        temperature;    -   AP is the temperature coefficient of the “beat”, which is        likewise determined during an initial calibration process;    -   t is the current temperature; and    -   t0 is the temperature at the time of initial calibration of the        scale.

All of the values in equation (1) are may be determined by themicroprocessor 208 or stored in memory associated with themicroprocessor 208. X and X0 may be determined indirectly from theoutputs of mixers 206 and 212, or, if desired, values of Ft and Fw maybe supplied directly (as shown by dotted lines in FIG. 6) to themicroprocessor 208. A few points are of note with respect to thevariables of equation (1). First, temperature coefficient AW isgenerally a composite of a number of environmental effects, includingthe characteristics of the piezosubstrate (typically lithium niobate) ofthe SAW transducer and the characteristics of the load cell material(typically aluminum). Second, at any specific time, the inverse slope Wfor all modes will be the same. Third, the weight P0 does notnecessarily occur at a transition from one mode (S=−1) to another mode(S=0), although it is shown that way in FIG. 7 for convenience. Fourth,mode 0 does not necessarily start where there is no weight on the scale(pan).

In practice, the SAW scales do not exactly follow equation (1). This isprimarily because the SAW IDT (transmitter and receiver of the SAW delayline, known as the Inter Digital Transducer) have different temperature(or environmental) zero shifts for different frequencies (i.e.,different weights on the scale) in response to environmental changes andaging processes. In one aspect, this effect is substantially linear,because of the range of frequency that is being used is kept small.

It should be appreciated that the frequency shift due to environmenteffects at zero will be the same as the frequency shift due toenvironment at the beginning point of each mode. For example, if thereis a shift of 1000 Hz at P0, there will also be a shift of exactly 1000Hz at P0+Dp and also at P0+2*Dp, etc. But there will not be an exactly1000 Hz shift at any other point along the graph of each mode. Anexample of this is seen in FIG. 8.

In the example of FIG. 8, the scale is arranged such that the frequencyof the delay line changes by 10 Hz per gram, and the beat period Dp is30,000 grams which corresponds to 300,000 Hz (300 khz). At temperaturet=T1, with zero weight on the platform (i.e., at P=P0), for mode #0, thefrequency of the delay line is 93.00 Mhz. The SAW oscillator can then beused to change the mode of measurement from mode #0 to mode #-1 using a“mode selector”. This can be accomplished by the “push oscillator” 216as previously described or by another means, such as a narrow bandfilter. For mode #-1, for the same zero weight on the platform, thefrequency of the delay line is 93.30 Mhz. If 30,000 grams are placed onthe platform of the scale while in mode #0, the frequency of the delayline will also be 93.30 Mhz.

At some later point in time, the calibration of the scale is checked atthe same temperature T1. It is found that for P=P0 (no weight on theplatform), for mode #0, the frequency of the delay line has stayed thesame, i.e., 93.00 Mhz. But for mode #-1, the frequency of the delay lineis 93.2998 Mhz—this is a zero shift of −200 Hz. For this example wherethe frequency of the delayline changes by 10 Hz per gram, a zero shiftof −200 Hz corresponds to a shift of (−)20 grams. This same shift of(−)20 g will be seen for every instance where the frequency of the delayline is in the 93.30 Mhz frequency range (i.e., for each mode). As aresult, the inverse slope W of the saw-tooth function shown in FIG. 8,is now changed to W′. This can be described by the equation:

W′=W*[1−(∂P/Dp)]  (2)

-   -   where ∂P is the change in weight measurement (i.e., old weight        minus new weight), here 20 grams.

In this example W′=W*[1−(20 g/30,000 g)]=W*[1−0.00066]=W*0.999333. Itshould be noted that in FIG. 8, the change in the inverse slope ishighly exaggerated for purposes of illustration.

Now, if W is replaced with W′ in equation (1), the value of P willchange for every weight for every mode. In this way, the initialcalibration for the scale has been corrected without use of any externalcalibration mass. The result of this recalibration is a significantimprovement of the overall accuracy of the scale. For example, thespecification for linearity for a five mode SAW scale was enhanced from1:20000 to 1:60000. Similarly, sensitivity drift was reduced to lessthan 1 ppm per 1° C. in a range 10° C.-40° C.

FIG. 9 is a flow chart implemented by the microprocessor 208 of FIG. 6with respect to the automatic recalibration aspect.

At step 1200, a determination is made by microprocessor 208 that theweight on the scale is not changing (i.e., either there is nothing onthe platform—a “null load”, or the weight on the platform is notchanging) for a desired period of time (e.g., one minute, or fiveminutes, or any other desired amount of time) and that the scale is in aparticular mode (denoted mode S for purposes of illustration).

At step 1210, a measurement of temperature t is made as is a measurementof X(f), where X(f)=Ft/Fw. Ft is the SAW reference sensor frequencyindication output by amplifier 204 of the delay line oscillation loop124, 126, 204. Fw is the SAW weight frequency indication output byamplifier 202 of the delay line oscillation loop 120 b, 122 b, 202.

At step 1212, a value for P is calculated with the scale in mode Saccording to equation (1) set forth above:P=[W*(X−X0)*{(N0+S)/N0}*AW*(t−t0)]+[Dp*S*AP*(t−t0)], where the variablesare as previously defined. These variables are stored in memory by themicroprocessor 208.

At step 1214, the delay line with amplifier 202 is caused to operate inan adjacent mode (i.e., mode S+1, or mode S−1), e.g., by causing thepush oscillator 216 to provide a different frequency that is injected bymodulator 220 and provided to amplifier 202.

At step 1216, a second value for P is calculated according to equation(1) for the scale in the adjacent mode. Then, at step 1218, the secondweight measurement is subtracted from first weight measurement to get aweight difference ∂P. With the calculated weight different, a modifiedinverse of the slope W′ is calculated at step 1220 according to equation(2) set forth above: W′=W*[1−(∂P/Dp)]. The calculated value of thisvariable W′ is stored in memory by the microprocessor 208.

At step 1222, the new inverse slope W′ is substituted (stored) as thenew value for the variable W as a recalibration. In other words, thevalue for the inverse slope variable W is updated with a new calculatedvalue of W′. Steps 1200-1222 may be repeated on a regular basis orwhenever the processor determines at 1200 that the weight on the scaleis static.

According to another aspect, and as previously mentioned, a lowresolution transducer or sensor (e.g., 18 a of FIG. 1) such as acapacitive transducer is utilized to help determine the mode in whichthe SAW scale is operating. This lower resolution secondary transduceris not very stable and its readings can drift with time and temperatureand physical conditions (e.g., if the scale is jostled). It the drift ofthe secondary transducer reading is large enough, its determination asto the mode in which the SAW transducer is operating may be incorrect,thereby causing a large error in the weight determination of the SAWscale. By way of example, if a fifty pound capacity scale operates infive modes (ten pounds per mode), and the SAW scale transducer providesa reading that indicates a weight of three pounds, and the secondarytransducer provides a reading that indicates a weight of fourteenpounds, the correct output of the scale would be thirteen pounds (i.e.,the scale is operating in the mode spanning ten to twenty pounds), andthe secondary transducer could be recalibrated to read thirteen pounds.On the other hand, if the secondary transducer has drifted considerablyand provides a reading of eight pounds, then it is not clear whether thecorrect output would be three or thirteen pounds with the scaleoperating in the mode spanning zero to ten pounds or ten to twentypounds; and if the drift was even more, say to six pounds, the scalewould be caused to output a reading of three pounds, even though thecorrect reading would be thirteen pounds.

In order to avoid incorrect readings resulting from secondary transducerdrift, upon initial calibration, a table of frequency differencesbetween modes is stored. While the frequency difference between eachadjacent mode is similar, it is not the same. This difference isutilized to establish the mode in which the scale is functioning. Moreparticularly, as previously described, the scale includes a “pushoscillator” (e.g., formed from an adjustable oscillator 216, a mixer218, and a modulator 220). The SAW transducer has a preferred range offrequencies for operation. This is typically the range where the lossesare the lowest. But if the delay line has enough gain, it can alsooscillate at alternative frequencies which would be the frequency ofoperation at modes adjacent to the current mode. As previouslydescribed, the push oscillator is a frequency synthesizer that can beprogrammed to output a frequency that is close the adjacent modefrequency. It this is injected into the delay line, it will cause thedelay line to jump to that adjacent mode. Now, using the previousexample of a fifty pound capacity scale operating in five modes, it maybe seen that the frequency difference between adjacent modes may bestored as a function of mode and weight in a table such as Table 1:

TABLE 1 Mode Starting Freq. Difference Mode # Weight (lbs) adjacentmodes (Hz) 0 0 233,250 1 10 233,834 2 20 234,422 3 30 235,012 4 40235,606As can be seen, the frequency between adjacent modes increases forhigher modes. Measuring this difference allows the scale to determine,through use of the SAW transducer only, in which mode it is operating,and therefore the exact weight on the platform of the scale. This weightvalue can now be used to recalibrate the secondary sensor and eliminateany drift that may have occurred since the most recent previouscalibration.

FIG. 17 shows a method incorporating the improvement of having the scaledetermine the correct mode and weight using the SAW transducer, the pushoscillator and the frequency difference table. At 1300, an (optional)determination is made as to whether the scale is being powered up. Ifyes, at 1305, the operating frequency of the SAW delay line (e.g., 93.0MHz) is allowed to regulate itself for a short period of time and beread by the microprocessor. Then, at 1310 push oscillator injects afrequency similar to, but different than the frequency of the SAW delayline (e.g., 93.2 MHz) so as to cause the SAW delay line to operate at anadjacent mode. At 1315, the new frequency of the SAW delay line is readagain. At 1320, the difference between the two frequencies is calculatedand at 1325, the calculated difference is compared to values stored in atable in order to determine what mode the scale was operating in beforethe injection of the push oscillator frequency. By way of example only,using Table 1, if the difference in frequencies is found to beapproximately 234,422 Hz, then the scale was operating in mode 2 uponpower up and was forced into mode 3. The determination of mode is thenused to determine the value of the weight on the scale as previouslydescribed herein. At 1330, the weight as determined based on the modemay be displayed. Then, at 1335, the push oscillator is used to injectthe original operating frequency (e.g., 93 MHz) to cause the SAW delayline to operate in its initial mode and at 1340, the secondary orauxiliary transducer can be recalibrated to agree with the weightdetermined by the SAW scale using only the SAW transducer.

It is noted that the process of having the scale establish its operatingfrequency, having the push oscillator inject a frequency and waiting forthe SAW delay line to equilibrate in a new mode, and then using the pushoscillator again to inject the original operating frequency andequilibrate in the initial mode may take a few seconds to complete.Because weight determinations are desired immediately, it is notnecessarily desirable to always use the SAW transducer by itself todetermine both the mode of operation and the exact weight on theplatform. Accordingly, this technique is particularly useful upon powerup when an immediate determination is not expected.

There have been described and illustrated herein several embodiments ofSAW scale improvements and related methods. While particular embodimentshave been described, it is not intended that the disclosure be limitedthereto, and it is intended that the invention be as broad in scope asthe art will allow and that the specification be read likewise. Thus,while particular frequency difference values were described as beingutilized, it will be understood that other values for these variableswill be specific for the particular scale. Further, it will beunderstood that equivalent parts may be used for the described elements.For example, any suitable processor may be used as the “microprocessor”.It will therefore be appreciated by those skilled in the art that yetother modifications could be made without deviating from the spirit andscope of the invention.

What is claimed is:
 1. A method of weighing a load, comprising: a)providing an electronic weighing apparatus having a displaceable elasticmember that receives a load is displaced by the load such that thedisplacement of said elastic member is related to the weight of theload, a surface acoustic wave (SAW) delay line that operates in aplurality of modes at a plurality of different frequencies, a pushoscillator, a processor coupled to the SAW delay line and the pushoscillator, and a stored table relating mode numbers to frequencydifferences between adjacent modes; b) using the processor to read afirst frequency of the SAW delay line operating in a first mode; c)causing the push oscillator to inject a frequency similar but differentthan the first frequency in order to cause the SAW delay line to operatein a second mode; d) using the processor to read a second frequency ofthe SAW delay line operating in the second mode; e) calculating adifference between said first frequency and said second frequency; f)using said difference and said stored table to determine the first modeof the SAW delay line; and g) using the first mode and the firstfrequency, generating a determination of a weight of the load.
 2. Themethod of claim 1, wherein the electronic weighing apparatus includes adisplay, and said method further comprises displaying said determinationof the weight of the load on said display.
 3. The method of claim 1,wherein the electronic weighing apparatus includes an auxiliarydisplacement sensor providing an auxiliary weight reading output, andsaid method further comprises recalibrating the auxiliary displacementsensor to said determination of the weight of the load.
 4. The method ofclaim 1, further comprising conducting b) through g) upon start-up ofsaid electronic weighing apparatus.